Those who wear prosthetic legs or feet confront several challenges to achieve natural and comfortable walking. These issues include changes in “effective heel height” and changes in the slope of the terrain encountered. Artificial limbs may function properly over smooth, horizontal surfaces or on slight inclines, but steeper inclines present problems, particularly when walking downhill. If the angle of the foot is not adequately adjustable, only the heel may make ground contact, making it difficult to accommodate the user's body weight and avoid buckling of the knee. For this reason, many prosthesis wearers often choose to walk sideways when walking downhill.
Manual, mechanical adjustments are often difficult to set or calibrate, and do not automatic adjust for changes in heel height or varying terrain slope. To address these deficiencies, various self-adjusting structures have been introduced over the years. One improvement has been in the development of damping mechanisms, some involving hydraulics. An early example is U.S. Pat. No. 2,470,480, which describes an artificial foot having hydraulic cushioning means between the ankle and the sole plate. A needle value is provided for controlling the passage of hydraulic fluid so that the walking action can be adapted to a particular person.
U.S. Pat. No. 6,855,170 resides in a foot connected to a leg prosthesis via a pivot axle such that the angular position between the foot and the leg prosthesis is adjustable to a desired angular position. This is accomplished with a piston displaceable in a cylinder attached to the leg prosthesis. A two-way valve permits the flow of medium between the two chambers in the cylinder. The two-way valve is adjustable with a control stick operable from the outside of the prosthesis. One problem with this design is that the hydraulic cylinder is oriented along the foot, resulting in a bulky structure.
To achieve a more compact design, U.S. Pat. No. 7,985,265 describes a prosthetic ankle and foot combination wherein the hydraulic cylinder is oriented along the leg as opposed to the foot. This device has an ankle joint mechanism constructed to allow damped rotational movement of a foot component relative to a shin component. The mechanism provides a continuous hydraulically damped range of ankle motion during walking with dynamically variable damping resistances, and with independent variation of damping resistances in the plantarflexion and dorsiflexion directions. Single and dual piston hydraulic damping arrangements are disclosed, including arrangements allowing independent heel-height adjustment.
Although the solution described in the '265 patent achieves a more compact package, it suffers from the same drawback as other prostheses that utilize a conventional hydraulic cylinder; namely, since the ankle rotates and the cylinder is straight, some form of linkage must be includes to convert pivoting motion to reciprocal. These linkages need pivot axes of their own, resulting in more moving parts and places where wear can occur.